Various surface properties of an integrated circuit or wafer are determined in a metrology system from the spectrum of light reflected from the integrated circuit surface. The spectrum of reflected light is referred to herein as the spectral reflectance or, equivalently, the reflectance spectrum. One surface property of interest, for example, is thin film thickness. Another property may be the length or height of a periodic structural features. The reflected light is collected and dispersed by a wavelength separation element, such as a diffraction grating. The wavelength dispersed image or spectrum obtained from the diffraction grating represents a distribution of light intensity across a wavelength range. This distribution is processed in accordance with conventional algorithms to produce a measurement of the surface property of interest. In order for such processing to produce valid results, the distribution or spectrum must represent the absolute reflectance spectrum of the integrated circuit surface. The light source used to generate the spectrum may itself have a non-uniform intensity distribution across the wavelength range, which distorts the measured reflectance spectrum of the integrated circuit surface, thereby preventing observation of the absolute reflectance spectrum. Furthermore, the spectral distribution and intensity of the light source may drift over time. In order to solve these problems, the conventional approach is to periodically replace the integrated circuit or production wafer with a reference sample whose absolute reflectance spectrum has been predetermined. The metrology system then measures the spectral reflectance of the reference sample. The measured reflectance of the reference sample is then compared with its predetermined absolute reflectance spectrum, to generate a correction function. This correction function accounts for non-uniformity in the light source spectrum and for light source drift. The correction function is then applied to the observed spectral reflectance of the production wafer to produce the absolute reflectance of the production wafer. This absolute reflectance is then processed to compute the true measure of the surface property of interest.
One limitation of the foregoing approach is that a series of production wafer measurements is followed by the measurement of a reference sample, in order to frequently generate a new correction function to guard against system drift. For each such reference sample measurement, a reference sample must replace a production wafer on the metrology system wafer support, which greatly reduces productivity. What is needed is a metrology system that does not require the periodic replacement of a production wafer with a reference sample, but which nevertheless guards against system drift.